Unstable communication

Doing a range test in X-CTU, between 2 XBee PRO Series 2 (ZNET 2.5 router/end device AT, version 1247, with destination address crossed one to the other) some times it will run stable (fast & all good packets) but suddenly it becomes slow and bad packets start to appear. Tested using 9600 & 115200 bps, arratic behaviour happens aswell.

Distance between modules is a couple of inches, tested with high gain antennas.
XBee modules are plugged into XBee Explorer to Laptop’s USB ports.
I’ve tested in other computers and behaviour is the same.

The problem is in the modules.

My project is temporarily suspended for this reason.
Can someone help me to have a stable and trusty connection?

First off, that code as not been supported or offered in many years. You would be better off upgrading the firmware on both modules to the current Zigbee enabled codes. Next, all mesh based code take longer to perform communications. That is the nature of the beast. You will need to make sure you have your data receive timeout for the range test set to about 4 seconds. Also set both radios to Unicast addressing.